Gene transfer of a therapeutic protein is a promising approach toward treatment of genetic disease. Adenoassociated[unreadable] viral (AAV) mediated in vivo transfer of a coagulation factor IX (F.IX) gene to muscle or liver of[unreadable] subjects with severe hemophilia B is currently being evaluated in clinical trials. A major concern of treatment[unreadable] is induction of CDS* cytotoxic T lymphocyte responses (CTL), which may cause elimination of transduced[unreadable] cells (muscle fibers or hepatocytes) expressing the transgene product. Interestingly, AAV gene transfer has[unreadable] been shown not to cause CTL responses to a number of transgene products, which may be due to immune[unreadable] tolerance or ignorance. We will determine requirements for tolerance induction to a human F.IX transgene[unreadable] product in mice, which will receive AAV-F.IX gene transfer followed by challenge with an adenoviral vector (a[unreadable] potent vector for induction of CTL) expressing the identical transgene. CTL responses will be measured by[unreadable] antigen-specific lysis of target cells and immunochemical analyses of transduced tissues. We will define[unreadable] requirements for tolerance induction (target organ, levels of transgene expression, systemic protein[unreadable] delivery). We hypothesize that AAV-mediated hepatic gene transfer resulting in systemic F.IX protein[unreadable] delivery can lead to immune tolerance thereby preventing CTL responses to F.IX. Furthermore, we[unreadable] hypothesize that endogenous expression of immunodominant F.IX T cell epitopes contributes to tolerance[unreadable] thereby reducing the risk of CTL responses. The effect of the underlying mutation in endogenously[unreadable] expressed F.IX on CTL responses will be tested in a series of transgenic mice expressing non-functional[unreadable] variants of human F.IX taken from the hemophilia B database. Finally, a series of experiments is; proposed to[unreadable] identify subsets of regulatory T cells that can suppress development of CTL responses to F.IX. These[unreadable] studies are based on the hypothesis that hepatic AAV-F.IX gene transfer induces regulatory CD4+ T cells[unreadable] capable of suppressing CTL to F.IX. A detailed characterization of these regulatory cells and the[unreadable] requirements for their induction will lead to improved gene transfer protocols and will be an experimental[unreadable] basis for approaches to augment regulatory T cell responses. Improved understanding of cellular[unreadable] mechanisms underlying immune responses or absence thereof should provide the basis for design of[unreadable] strategies that preclude destruction of cells receiving viral gene transfer.